1. Field of the Invention.
This invention has to do with a significant discovery in the field of metallurgy. More particularly the invention is concerned with obtaining improved performance from superalloy components through the provision of diffusion coatings capable of multiplying their presently expectable service life. The invention finds its novelty in diffusioncoated superalloy components, and methods for their preparation.
The invention in its essentials is a deparature from previous efforts at improved diffusion coatings for superalloys which have been directed at ever more durable oxides of metals, in that, in large measure, the benefits of the invention flow from the rejection of oxygen by rhodium, that is, its characteristic inability to form stable oxides at expected service temperatures, e.g. above 900.degree.C.
The products of the invention have been occasioned by the inexorable advance of jet engine design in a quest for more efficiency, higher thrust and longer service life. The former requirements are achievable by use of increased temperatures within the hot section of the engine, but increased use temperature accelerates those processes which in time degrade the engine components, i.e. processes such as corrosion and thermal fatigue, thus shortening service life.
2. Prior Art.
Critical components of jet engine hot sections such as turbine blades, nozzle guide vanes, burner components and like structures are cast, forged, machined, or otherwise fabricated of superalloy materials, i.e. materials in which the base metal is nickel or cobalt to provide a "nickel base" or "cobalt base" alloy, respectively, which is characterized by great strength at elevated temperatures. These structures are ofen diffusion-coated with a material typically and predominantly aluminum, which characteristically forms a binary Ni-Al or Co-Al alloy, known as a nickel aluminide or cobalt aluminide alloy, with the superalloy nickel or cobalt base metal, respectively. Aluminum has the capacity to form highly stable, even refractory, oxide layers at the operating temperature of hot section components which are highly adherent and cohesive and thus effective to block incursions of corrosive chemical agents into the superalloy structure, so long as the Al.sub.2 O.sub.3 layer is intact. However, disruption of the aluminum oxide layer is followed by further oxidation and, subsequently, corrosion of the superalloy.
The aluminum oxide layer is subject to mechanical stress by the different rates of thermal expansion and contraction of the superalloy substrate and the diffusionproduced aluminide layer lying under the oxide layer. This stress may be aggravated by loss of base metal cations through the diffusion coating which leaves incomplete, and thus stressed, base metal crystal lattices beneath the diffusion coating.
As indicated, workers in the art have sought to incorporate one or another material into diffusion coatings to form more thermally stable or more tenaciously adherent corrosion-resistant surfaces on superalloy structures.
Improvement made this way has been incremental and has not contributed significantly to extending the life of components beyond the present aluminide coating.
One problem has been the migration of aluminum into the base metal structure and from the coating. This migration decreases the amount of aluminum available in the diffusion coating to form the refractory oxide which is the principal mechanism of superalloy structure protection. Limiting aluminum migration inward from the coating will greatly prolong coating life and correspondingly extend the service life of coated parts such as turbine blades.